When selecting a thermoplastic material for the development and manufacture of injection mold plastic parts, one must consider both the application requirements and the manufacturability of the selected material. One critical consideration when selecting a material for manufacturability is determining the “injection moldability”—how that material will flow or mold within the plasticating injection unit. To determine injection moldability, measurements that quantify the characteristics of that material are essential. Such characteristics include pressure per inch of flow, development of a frozen layer, flow rate, viscosity, shear rate, which all help to paint an accurate picture of how a material will actually flow or mold within the plasticating injection unit.
There are three methods that are commonly used measuring injection moldability. The most common is the Melt Flow Index (MFI) test due to its low cost, but it provides the most limited and irrelevant results in regards to its application for understanding how a melt will flow in an injection mold. A second method is a capillary rheometer, which is designed to capture the non-Newtonian viscosity characteristics of a thermoplastic material under a wide range of shear rates that the MFI test lacks. Both of these methods are highly limited because they are isothermal extrusion tests which will not capture the significant influence of a relatively cold mold acting to cool a flowing material as it is flowing through the mold's channels.
To the molder and the processor, the detailed viscosity vs. shear rate data provided by a capillary rheometer is fairly abstract and does not provide much value. The data is provided as viscosity vs. shear rate, possibly at multiple melt temperatures. Common viscosity units include psi-sec and pascal-sec. Shear rate is expressed as reciprocal seconds (sec^−1 or 1/sec). In either case, the viscosity and shear rate is expressed in somewhat abstract units that are not helpful to the product developer or molder. They are interested in how much pressure it will take to fill a mold having varied runner and part forming cavity geometries used to produce a given part. Therefore even if the test method was non-isothermal, the expressions of viscosity and shear rate do not provide meaningful information. As a result this data by itself is rarely used by those in product development or manufacturing.
The third common method used to evaluate how a plastic material will flow through an injection mold is the use of injection molding simulation programs. Injection molding simulation programs utilize the classic viscosity vs. shear vs. temperature data derived from a capillary rheometer and combines this with other material characterization tests to attempt to mathematically model the materials flow characteristics flowing through a relatively cold mold. There are numerous challenges to this mathematical modeling of which a significant part is the material characterization methods on which the programs are dependant. These characteristics can include thermal conductively, melt and solid phase densities, pressure-specific volume-temperature characteristics (P-V-T), specific heat, etc. Many of these are variables that cannot be accurately measured under the conditions that exist in injection molding. An example would be P-V-T data which is captured while temperature changes are commonly only 3° C. per minute. This is in contrast to actual performance where a thermoplastic material forms a frozen skin when the plastic experiences temperature drops from a molten temperature which is commonly about 250° C. in the plasticating injection unit to what is commonly less than 50° C. in a flow channel in the mold in small fractions of a second. These actual cooling rates are therefore in the range of a few hundred degrees per second to more than a thousand degrees per second. As a result of the complex characteristics of plastic materials as they flow through the mold and the difficulty in modeling these characteristic for use in an injection molding mold filling simulation program, there is an inherent error in the ability of molding simulation programs to accurately predict mold filling pressures. Predicting mold filling pressures are very important to the users of injection molding simulation programs and the plastics injection molding industry as a whole.
Other methods have been developed to evaluate the viscosity characteristics of a plastic material as influenced by non-isothermal conditions. One such device disclosed in PCT Publication Number WO 2012/038769 does this by using a mold with a measuring capillary channel in the moving half of the mold and sensors in the stationary half of the mold. The capillary channel opens at one end to allow the material flowing through the measurement channel to flow through the capillary to atmosphere. The injection rate is controlled and determined by an injection molding machine that injects the material through the capillary section of the mold. Melt temperatures and pressures are measured with sensors. Shear rates can be calculated by assuming the flow rate from the injection molding machine to be the same as that flowing through the capillary and knowing the channel cross section. Shear stress can be calculated by knowing pressures and channel cross sectional shape. Calculated shear rate and shear stress can then be used to calculate viscosity. The mold can be cooled to run isothermal rheological tests and the mold opened to remove the sample material after it has solidified. The capillary channels are in mold inserts that can be removed by opening the mold, unscrewing attachment bolts, and replacing the capillary channel insert with an alternate insert with another capillary geometry enabling the study of various viscosity characteristics and applying various corrections factors commonly required and applied to the rheological characterization of a plastic material. Though this method is an improvement over many other rheological flow characterization methods, it does not address a number of fundamental needs of the injection molding industry addressed by the qualities of the methods of the invention disclosed herein.
U.S. Pat. No. 5,076,096 to Blyler, Jr. et al. (hereinafter “Blyler”) discloses a process and apparatus for measuring viscosity of a thermosetting composition, which has some of the same limited utility as discussed above. The Blyler apparatus is also limited in its applicability to materials that are thermosetting compositions. Thermosetting materials require a relatively cool polymer to be injected into a relatively hot mold, the hot mold accelerating a chemical reaction where the thermosetting material will cross link. This is very different than the conditions required for thermoplastic materials. Thus using any other kind of material would be beyond the limitations of what is claimed and taught.
Blyler also discloses two different modes of measuring the viscosity of a thermosetting: an isothermal mode and a non-isothermal mode. In the isothermal mode, the preheated thermosetting resin is placed in a heated reservoir where it becomes a fluid and is forced through the mold and capillary. The thermosetting resin, reservoir, and mold are all heated to the same temperature (See Blyler col. 4, lines 15-22). In the non-isothermal mode, the thermosetting resin is preheated to a temperature lower than the temperature of the mold (See Blyler col. 5, lines 42-45). In this non-isothermal mode, the thermosetting material flows through a hot mold runner and capillary. Therefore, neither mode discloses that the thermosetting is heated at a temperature higher than the temperature of the mold through which it will flow. Measuring thermosetting material flows at low isothermal temperature conditions causes inaccuracies in the measurements because the thermosetting material will not be able to initiate the normal chemical reaction to begin cure in the flowing material nor form a solidifying skin as it flows through the capillary, which is much more likely to occur under non-isothermal flow conditions using a relatively hot mold.
Other prior art methods and apparatuses make measurements of material flow conditions prior to the material entering a flow channel through which the material ultimately flows. Taking measurements prior to the material entering a flow channel creates greater error in the calculations of the characteristics of the material. The measurements taken are only a prediction of how the material will flow through the flow channel and mathematics and/or a multitude of additional tests must be used to make adjustments for any real world factors that cannot be calculated based on those predictions. Using mathematical calculation corrections, sometimes known as fudge factors, will always have some level of inaccuracy because there are always variables that cannot be factored by mathematical equations. Common mathematical calculation corrections used in these circumstances are the Bagley Correction or the Rabinowitsch Correction.
U.S. Pat. No. 6,023,962 to Wang et al. (hereinafter “Wang”) for example discloses a process and apparatus for testing the rheological properties of resins. Wang discloses that the apparatus comprises a mold having a mold cavity that resin first enters into, a slit, a thermocouple device positioned within the cavity near the slit, and a pressure transducer within the reservoir or reservoir walls (see Wang col. 4, lines 48-67). To measure the rheological properties of the resin, the pre-heated liquid resin enters into and is heated within the mold cavity to a high temperature and, once heated, is then extruded through the slit, while the pressure transducer or thermocouple device measures the pressure within the reservoir (See Wang col. 8, lines 45-60). Therefore, the measurements of the resin are also taken prior to the resin's entering of the slit and not while the resin is flowing through the slit.
It should be noted that in the embodiment of the apparatus that discloses multiple reservoirs on a single mold plate, each reservoir is filled with resin simultaneously and in turn the resin flows through the slit connected to each reservoir so that one can obtain viscosity data at different shear rates simultaneously, as opposed to flowing the resin each through slit in sequence (see Wang col. 6, lines 49-57). This configuration requires a multitude of sensors for each reservoir. Each of the multitude of reservoirs require individual loading of material into each reservoir. Also the different pressures required to drive the fluid thermosetting material simultaneously through the multiple strips could create numerous mechanical problems due to uneven distributions of pressure and force requirements of what may be assumed to be a primary injection ram driving all of the multitude of pistons for each of the reservoirs.
Other prior art methods and devices are used for monitoring, documentation, or control of specific injection molding machines that are in operation to manufacture an injection molded plastic product. These methods and devices are not used for a general understanding or characterization of the material, but to check on the performance of a plasticating injection unit producing a commercial product. U.S. Pat. No. 8,329,075 to Bader (hereinafter “Bader”) for example discloses a method for determining the viscosity of a material in an injection mold of an injection molding machine. The method is particular to viscosity determinations within a specific injection molding machine being used to manufacture a specific product in a commercial mold. The shear rate and shear stress values calculated for viscosity calculations are actually not true values of shear rate, shear stress, or viscosity as the channel cross sections of the cavity producing the commercial part will continually vary. Therefore the values calculated are the culmination of a wide range of actual viscosity conditions that will occur in the melt as it flows through these varied geometries. This is very different from methods whose purpose is to characterize the flow characteristics of a polymer. The subject matter does not apply to directly quantifying the characteristics of materials themselves, but rather is intended to “monitor, and possibly control, an injection molding process under practical conditions” (see Bader col. 2, lines 38-40).
The use of classic fundamental rheological characterization models of a plastic materials has limited use to those in the injection molding industry. Classical rheological characterization data does not provide an accurate means to determine how a plastic material will fill a mold without the use of complex and expensive mold filling simulation programs. The economics of an accurate material characterization test is an important consideration for the industry. With well over 100,000 variations of commercial thermoplastic materials, the cost to characterize a material must be kept to a minimum without compromising the quality and usability of the data. These variations in polymer materials are not limited to polymer type. A plastics material's mold filling characteristics are dependent on numerous variables including material type, blends of materials, copolymers, molecular weight, molecular weight distribution, additive types, and percentage of additives. Under non-isothermal conditions of the injection molding process a rheological value is not relevant. The resistance to flow through a runner or cavity channel will vary not only with the flow rate and temperature of the material, but also with the cross sectional size and thickness of the channels. This is in contrast to isothermal tests where the rheological characteristics are constant regardless of cross sectional dimensions. It is typical that the rheological characterization of a given material be conducted on ether a MFI with a single diameter capillary or on a capillary rheometer with a single diameter capillary. This is not characteristic of what would happen in non-isothermal conditions where the cross section will influence the heat exchange between the molten material and channel boundaries. There is also a need in the industry to provide an economical means to verify the accuracy of mold filling simulation programs in simulating the flow and pressure conditions through the geometry of a mold being analyzed.
By relying on calculations based on the injection rate of an injection unit, prior art methods described above do not accurately capture the material flow rates, velocities, and shear rates through a flow measurement channel. There are a number of sources of error in these methods. One is the fact that melted plastic material is compressible under the high pressure experienced during actual injection molding. This compression can be over 20%. As a result the flow rate as determined by the velocity of the injection piston or screw used to inject molten material through a flow measurement channel will not be the same as the actual flow rate through the flow measurement channel. Additionally there can be leakage of molten material past the injecting piston or screw. In plasticating injection units that utilize an injection molding screw, there can be a number of additional issues which will cause the indicated injection rate determined by screw advancement velocity to be a poor indicator of actual injection rate through a downstream flow channel. One is that in practice, after a charge of material has been developed for injection, the screw is pulled back (generally referred to as suck back) so that the material does not drool out of the machines nozzle. This fully decompresses the molten material and, depending on the amount of suck back, will vary the actual amount of material being injected into a mold once the screw moves forward to drive the molten material into the mold. Additionally plasticating injection units typically use some form of check ring or check valve to prevent back flow over the screw flights during injection when the screw is driven forward. During plastification and from suck back, these valves are in a forward open position. As the screw advances forward to inject the molten material, the valves will move backward in order to seat and thereby help prevent molten material from leaking back into the screw flights. This shift will influence actual flow rate. Finally, there is expected to be some leakage over these check rings during injection molding. All combined, there can be significant variation between flow rate expected based on the screws injection velocity and the actual flow rate through the mold or a melt measurement channel.
Unlike the prior art methods and apparatus discussed above, what is presented is a new method and apparatus for quantifying the characteristics of a flowing thermoplastic material melted into a fluid state by a plasticating injection unit. The melted thermoplastic material is injected at different flow rates through flowing material characterization channels having cross-sections of different geometries. The characteristics of the material are measured within these flowing material characterization channels. This method and apparatus determines the characteristics of the material itself, not just the characteristics of the material in a specific injection molding machine.